Atmospheric scattering of light in the visible and near-visible spectral regions influences the earth's climate by changing short wave radiative forcing and influences optical remote sensing by ground-based, airborne, and satellite systems, including visual perception by humans. Scattering by particle free air is due to its gaseous components and seems to be well understood. The contribution of suspended particles to atmospheric scattering is highly variable in space and time and generally dominates the total extinction in the visible near the earth's surface.
Particle scattering is commonly characterized by integrating nephelometers that measure the scattering component of extinction also known as the total scattering coefficient. The necessary integration of the scattered light over all angles (i.e., 4π) is performed geometrically with one of two schemes devised by Beuttell. Either a cosine-law diffuse light source is used to illuminate the scattering volume that is viewed with a detector, or detector and light source are reversed and a parallel light beam is used and the scattered light is detected by a cosine detector. The first arrangement is more common, and the second arrangement is known as reciprocal nephelometer. Calibration is generally performed with two gases that have a large difference in scattering coefficients such as air and carbon dioxide.
An ideal integrating nephelometer provides, after calibration, a direct measurement of the total scattering coefficient of suspended particles independent of their properties such as size, composition, and physical state. Real integrating nephelometers fall short of this ideal mostly due to imperfect angular and wavelength response and imperfect particle sampling.
Imperfect angular response arises because illumination intensity or detector sensitivity should be cosine-weighted but is not. In particular, light scattered at angles smaller than about 7° and larger than about 170° is not detected by modern nephelometers. These angles are known as truncation angles. As the angular distribution of particle scattering is strongly dependent on particle size, the measured scattering coefficient depends on this property. For the special case of very large particles, at least half of the scattered light is due to diffraction that is scattered in near-forward direction and may not be detected.
Imperfect aerosol sampling results in an aerosol in the measurement volume of the nephelometer has different scattering properties than the ambient aerosol of interest. Sampling losses of large particles due to impaction and gravitational settling are common.
Imperfect wavelength response is due to the integration of particle scattering over a wavelength range, typically 40 nm for commercial nephelometers using a thermal light source. As the wavelength dependence of particle scattering is size and refractive index dependent, the measured scattering coefficient is not appropriate for the nominal wavelength but only for the wavelength range with appropriate weighting due to the combined spectral response of light source, detector, filter, and other optical elements.